pytest — Naming Rules & Discovery

• Know pytest's discovery rules for files, functions and classes.
• Lay out a project with a tests/ folder.
• Use conftest.py for shared setup.
• Configure pytest in pyproject.toml / pytest.ini.

files        test_*.py   OR   *_test.py
functions    test_*
classes      Test*       (no __init__, no TestCase needed)
methods      test_*   inside a Test* class

Plain functions OR Test classes

# both are discovered by pytest:

def test_add():                 # plain function
    assert add(2, 3) == 5

class TestMath:                 # class — name starts with "Test", NO __init__
    def test_add(self):
        assert add(2, 3) == 5
    def test_sub(self):
        assert sub(5, 2) == 3

pytest classes group related tests but (unlike unittest) need no base class and no __init__.

The standard layout

myproject/
├─ src/myproject/
│   ├─ __init__.py
│   └─ pricing.py
├─ tests/
│   ├─ test_pricing.py
│   └─ conftest.py        ← shared fixtures (next lessons)
└─ pyproject.toml         ← pytest config

conftest.py — shared setup, auto-loaded

# tests/conftest.py — pytest imports this automatically
import pytest

@pytest.fixture
def sample_user():
    return {"name": "Aisyah", "age": 13}

# any test in this folder can just ask for 'sample_user' as an argument

conftest.py is special: pytest loads it automatically and shares its fixtures with every test in that directory. No imports needed. (Fixtures themselves are Lessons 14-15.)

Config in pyproject.toml

# pyproject.toml
[tool.pytest.ini_options]
testpaths = ["tests"]
addopts = "-v --strict-markers"
python_files = "test_*.py"

This tells pytest where to look and which flags to always apply — so a bare pytest just works for everyone on the team.

The import gotcha

Two test files both named test_utils.py in different folders
without __init__.py → pytest errors with "import file mismatch".
Fix: add __init__.py files, or use unique test file names.

shop/
├─ shop/
│   ├─ __init__.py
│   └─ cart.py
├─ tests/
│   ├─ conftest.py
│   └─ test_cart.py
└─ pyproject.toml

# tests/conftest.py
import pytest
from shop.cart import Cart

@pytest.fixture
def empty_cart():
    return Cart()

@pytest.fixture
def cart_with_items():
    c = Cart()
    c.add("roti", 2, 1.50)
    c.add("milo", 1, 3.00)
    return c

# tests/test_cart.py — fixtures arrive as arguments
def test_new_cart_is_empty(empty_cart):
    assert empty_cart.total() == 0

def test_cart_total(cart_with_items):
    assert cart_with_items.total() == 6.00

class TestCartEdgeCases:
    def test_remove_missing_item(self, cart_with_items):
        import pytest
        with pytest.raises(KeyError):
            cart_with_items.remove("nasi")

$ pytest            # config + conftest mean a bare command just works
tests/test_cart.py::test_new_cart_is_empty PASSED
tests/test_cart.py::test_cart_total PASSED
tests/test_cart.py::TestCartEdgeCases::test_remove_missing_item PASSED
3 passed

Read the diff

The fixtures live in conftest.py and tests just name them as arguments — pytest injects them. The pyproject.toml config means a teammate types pytest and everything runs the same way. Both a plain function and a Test* class coexist. This is the canonical professional layout.

pytest discovers test_*.py files, test_* functions, and methods inside Test* classes (no base class needed). Keep tests in tests/; put shared fixtures in conftest.py (auto-loaded); configure once in pyproject.toml. When a test doesn't run, check the names first — use pytest --collect-only to see what was found. Next: the smart assert introspection in detail.